In recent years, formic acid, similarly to methanol and organic hydrides, is being studied all over the world as a promising hydrogen-storing material, because of its ability to produce hydrogen at room temperature. Formic acid fuel cells (for example, see Patent Document 1) have already been begun to be supplied to the market.
For production of hydrogen from formic acid in production of fuel cells and the like, there is a method using a microorganism that generates hydrogen using formic acid as the energy source (biocatalyst) (Patent Document 2). However, methods using chemical catalysts, which are excellent in heat resistance, acid resistance and the like compared to biocatalysts, are being extensively developed at home and abroad. Representative examples of the methods using chemical catalysts include the high-temperature pyrolysis method (Non-patent Document 1), solid catalyst method (Non-patent Document 2) and metal complex catalyst method (Patent Documents 3 to 5, Non-patent Documents 3 to 7).
On the other hand, as iridium hydride complexes represented by the general formula IrH3(CO)L2, compounds having PPh3, PCy3, PEt3, PPh2(4-MePh) and/or PEt2Ph [wherein Ph is an abbreviation for a phenyl group, Cy is an abbreviation for a cyclohexyl group, Et is an abbreviation for an ethyl group, and Me is an abbreviation for a methyl group; the same applies hereinafter] as a ligand Ls are known (according to search with Scifinder). Further, as iridium complexes represented by the general formula Ir(acac)L2 [wherein acac is an abbreviation for acetylacetonato; the same applies hereinafter], compounds having PPh3, PMePh2, PMe3 and/or PPh2Py [wherein Py is an abbreviation of a pyridyl group; the same applies hereinafter] as a ligand L(s) are known, and, as iridium hydride complexes represented by the general formula IrH3L3, compounds having PPh3, PMe2Ph, PPh(4-MePh) and/or PMePh2 as a ligand Ls are known.
For example, in relation to such compounds, Patent Document 6 describes a method wherein, by contacting IrHxHal3-xPn2 or IrHxHal3-x(CO)Pn2 (wherein x represents 0 to 3, Pn represents tertiary phosphine or arsine, and Hal represents Cl, Br or I) with a mixture of formic acid and another fatty acid or fatty acid ester, formic acid in the mixture is selectively decomposed. The general description in its specification describes that the above complex is preferably IrH3P(PAr3)3 (wherein Ar represents a phenyl group or a substituted phenyl group) (see lines 59-60 in column 1, and claim 5), and IrH3(PPh3)3 is used in Examples (see Examples 5, 6, 8 and 9).
However, Patent Document 6 only describes that formic acid at a low concentration can be decomposed in a mixture in which a fatty acid or fatty acid ester coexists (which can also be said to be a formic acid solution containing a fatty acid or fatty acid ester as a solvent). The specification describes that the concentration of formic acid in the above mixture is, for example, 1 to 50% (see the 3rd line from the bottom to the final line in column 1, and claim 7), but the concentration was about 3 to 10% in Examples wherein IrH3(PPh3)3 was used (see Examples 5, 6, 8 and 9). Moreover, neither Example in which a complex having a “substituted phenyl group” was used as a complex represented by IrH3P(PAr3)3 nor Example in which a complex represented by IrH3(CO)Pn2 was used is disclosed at all.
Further, Patent Document 7 describes a method for producing a formic acid or formic acid ester, wherein, in the presence of a catalyst containing a hydride complex of a group VIII transition metal and an aliphatic tertiary amine, a compound represented by the general formula ROH (wherein R represents hydrogen or a hydrocarbon group) (that is, water or an alcohol) is reacted with carbon dioxide and hydrogen. It is described that specific examples of the hydride complex of a group VII transition metal include (PPh3)4RuH2, (PPh3)4IrH3 and (PPh3)3(CO)RhH (see the upper right column in page 2). Further, in Examples, modes wherein a predetermined hydride complex is fed together with methanol, ethanol or water, and carbon dioxide and hydrogen are injected, to produce methyl formate, ethyl formate or formic acid, respectively, are described.
However, although the method described in Patent Document 7 is a method wherein the predetermined complex is used with hydrogen and the like fed separately to produce formic acid or the like (in this process, “hydrogenation of carbon dioxide” occurs), there is neither description nor suggestion on a method wherein hydrogen is obtained from another compound using the above complex and the obtained hydrogen is used to hydrogenate an unsaturated compound (compound having a carbon-carbon unsaturated bond and/or the like). Moreover, Patent Document 7 does not specifically disclose the hydride complex of a group VII transition metal other than those using triphenylphosphine as a ligand. That is, those using phosphine having a substituent other than a phenyl group as a ligand is not specifically disclosed. Moreover, in the hydrogenation reaction described in Patent Document 7, combined use of an aliphatic tertiary amine is described as an essential requirement.
Non-patent Document 8 describes a method wherein formic acid (and, preferably, a formic acid salt) is used in the presence of a transition metal complex to hydrogenate (reduce) olefin and acetylene. It is described that specific examples of the transition metal complex used for hydrogenation of olefin (1-octene) include (Ph3P)2Ir(CO)Br, (Ph3P)3Ir(CO)H, (Ph3P)2IrH2Cl and (Ph3P)2Ir(CO)2H (Table I).
However, Non-patent Document 8 does not specifically disclose a transition metal (e.g., iridium) complex represented by the general formula MHm(CO)Ln other than those using triphenylphosphine as a ligand. That is, those using phosphine having a substituent other than a phenyl group as a ligand is not specifically disclosed. Moreover, for unsaturated compounds having a carbon-carbon triple bond such as 3-hexyne and phenylacetylene, the document does not specifically disclose a mode other than one using (Ph3P)3RhCl as a transition metal complex. Further, in cases where the reaction is carried out at a temperature of about 60° C., especially in cases where only formic acid is used and no formic acid salt is used in combination, the yield of the hydrogenated product tends to be low.